The present invention relates to a multi-component oxide catalyst. Catalysts of this type are known to be useful in the oxidation of olefins to oxygenated hydrocarbons, oxidation of olefin-ammonia mixtures to unsaturated nitriles and oxidative dehydrogenation of monoolefins to diolefins. Examples of these reactions are the oxidation of propylene to acrolein, isobutylene to methacrolein, acrolein to acrylic acid, propylene and ammonia to acrylonitrile, and butene-1 or butene-2 to butadiene.
Examples of such catalysts may be found in U.S. Pat. Nos. 3,642,930, 3,773,692, 4,042,533 and 3,956,181. Basically, the present invention applies to any supported oxide catalyst utilized in the above reactions.
These catalysts are normally supported on some form of carrier to improve their physical properties. Typical carriers have included silica, titania, alumina and silicon carbide. The carriers are generally inert, but can be in the active form.
U.S. Pat. No. 4,092,354 discloses a conventional method of preparing a supported oxide catalyst. First, the required elements in the form of nitrates, etc., are dissolved in water. The carrier is then added, the mixture evaporated and calcined. The catalyst may be formed and calcined prior to carrier addition, as shown in U.S. Pat. Nos. 3,992,419 and 4,075,127.
Of the carriers noted above, silica has been one of the more frequently used supports. U.S. Pat. No. 4,092,354 discloses the use of silica sol, silica jel or silica containing diatomaceous earth. U.S. Pat. No. 3,773,692 discloses the use of fumed silica (silicon dioxide) along with one or more components having a small specific surface area. This combination of surface area carriers are used to control the activity of the catalyst.
It has been discovered that the use of fumed silica in the first step of a two-step catalyst preparation, followed by the addition of a standard high surface area silica compound in the second step, results in a catalyst having better physical strength for a fluid bed or transfer line operation and greater activity and selectivity than the prior art supported catalyst.